1. Field of the Invention
This invention relates to an ultrasonic lithotresis apparatus for directing ultrasonic shock waves from the outside of a body to a diseased portion in the body, and treating the diseased portion, for example, by breaking a renal calculus or kidney stone.
2. Description of the Related Art
As prior art there are known an ultrasonic pulse apparatus for breaking renal calculi disclosed in Japanese laid-open patent application No. 60-145131, and an apparatus for examining and locating tumors using ultrasound disclosed in U.S. Pat. No. 4,658,828.
In these apparatus a large number of piezoelectric elements are arranged in a mosaic pattern on a spherical surface, thus constituting a transducer assembly. With the transducer applied to a subject (patient) through a liquid bag filled with an ultrasonic medium such as water, ultrasonic shock waves are transmitted by the piezoelectric elements so as to concentrate or converge on a diseased portion in the kidney, namely, a calculus, thereby breaking the calculus.
The ultrasonic shock waves are generated by applying a driving impulse voltage to each of the piezoelectric elements from a driving circuit.
In order to locate the calculus a mechanical scanning type ultrasonic probe is mounted on the transducer assembly. An ultrasonic diagnosis apparatus is coupled to the probe.
In view of conversion efficiency the piezoelectric element is formed of an inorganic piezoelectric material such as lead zirconate titanate (PZT) or an organic high polymer piezoelectric material such as PVDF. The piezoelectric element is usually driven at its resonance frequency because of its impedance characteristics. The impedance characteristics are as shown in FIG. 1.
As shown, the piezoelectric element has a considerably high impedance at the frequency range without the resonance frequency. The piezoelectric element has a low impedance and high Q at the resonance frequency. The piezoelectric element vibrates in a very narrow bandwidth centered at the resonance frequency fr. Thus, even if a driving impulse voltage as shown in FIG. 2 is applied to an element, an ultrasonic shock wave generated by the element exhibits a waveform Pb which vibrates positive and negative with respect to a sound pressure level of 0 as shown in FIG. 3. This means that negative amplitude, i.e. negative sound pressure is generated. The negative sound pressure creates vacuum bubbles in the body fluid. Since the surrounding tissues contract as the bubbles disappear, there is the possibility of destroying the living tissues. This phenomenon is generally called "cavitation."
The vibrational waveform Pb does not coincide with the driving pulse waveform Pa.
The frequency spectrum of the vibrational waveform Pb is near to the mono-spectrum consisting of a single frequency component, and its amplitude (peak sound pressure) Ab becomes small as compared with driving pulse Pa. Thus, energy of the ultrasonic shock wave becomes small as well. Further, the vacuum bubbles created by the negative sound pressure absorb the energy. As a result, the ultrasonic shock waves will be attenuated before reaching an object such as the calculus, thus causing the possibility of insufficient break of the calculus.
Accordingly, it is proposed that a damping material is laminated on the rear side of the transducer in order to make the Q of the transducer extremely small and thus broaden the frequency range (bandwidth) of vibration. Owing to the damping material a shock wave can be generated which is almost similar in waveform to the driving waveform and wherein has small negative sound pressure On the other hand, however, the damping material adversely affects the conversion efficiency.
Further, it is proposed that the transducer is driven at a frequency in a frequency range below the resonance frequency that is flat in the impedance characteristics so as to realize a vibrational waveform close to an optimum waveform. However, by the use of the frequency range in which the Q of the transducer is small the conversion efficiency is still poor.